In the last decades, the search for recyclable feed-stocks has increased considerably as crude oil based products are expected to become more limited in the future. One of these promising feed-stocks is polystyrene. Due to its widespread availability, especially as recycled or reclaimed material, polystyrene has become more and more attractive for industries which require large quantities, such as coatings and inks industries.
From the viewpoint of environmental protection and conservation of resources, it is desirable to use recycled materials as much as possible. However, currently, only a portion of polystyrene waste is recycled, and it would be advantageous to use more of recycled polystyrene in new products.
The high molecular weight of polystyrene and absence of residual monomer content, without any risk of monomer migration, and its outstanding organoleptic properties, makes polystyrene an ideal resin for packaging. Polystyrene is particularly well-suited to food packaging.
However, the use of polystyrene in a dissolved, applicable form in, for example, overprint varnishes for packaging goods or inks, is difficult, as many solvents which dissolve polystyrene are hazardous, highly flammable, volatile organic compounds. For example, toluene, xylene, acetone, methyl ethylketone, and ethyl acetate are often used to dissolve polystyrene. These solvents are difficult to handle in print shops, and difficult to use, especially for food packaging.
For example, E. Flick describes toluene based polystyrene varnishes for coatings applied by gravure cylinders (“Printing ink formulations”, ISBN 0-8155-1014-4, page 64). For paintings, varnishes in turpentine are proposed in UA 82856 and LT3623. Turpentine and limonene, which are described in the literature to be able to dissolve polystyrene, are less suitable due to a low flash point and strong odor.
Polystyrene paints in xylene are described in RU2293096 and RU 2472827. A varnish of polystyrene based on xylene and acetone is disclosed in MX2013003779. Tetrahydrofurane as solvent for polystyrene is described in U.S. Pat. No. 5,278,282. U.S. Pat. No. 4,517,312 describes chlorinated solvents such as methylene dichloride as a solvent for polystyrene, which is non-flammable, but highly toxic.
Besides their highly flammable, toxic and odorous character, the aforementioned solvents are moreover not suitable for a lithographic sheet-fed or heat-set printing process. Many substrates for packaging are printed with a sheet-fed ink applied by a lithographic process. This printing process requires a high boiling point solvent, having a boiling point usually >250° C., which may not evaporate during the printing process. U.S. Pat. No. 5,629,352 describes solutions of polystyrene in high-boiling glycol ethers as co-solvents in a process to purify polystyrene, but such polar materials are less suitable for use as solvents in a lithographic process due to the high risk of over-emulsification.
Moreover, polystyrene also has poor or no solubility in solvents typically used in packaging printing inks and varnishes applied by sheet-fed or heat-set printing, such as high boiling mineral oils, such as, for example, 6/9 test oil from Halterman company, and vegetables oils such as, for example, linseed oil, soybean oil, castor oil. In addition, polystyrene exhibits very limited compatibility with typical resins used in graphic arts such as phenolic rosin resins, maleic modified rosin resins, alkyd resins and rosin salts (resinates). This makes it very difficult to introduce polystyrene or preferred recycled polystyrene into graphic arts products, and especially difficult for the lithographic printing process, as there is no suitable solvent. Finally, in WO03/035729, rapeseed methyl ester is mentioned among other solvents as a co-solvent in a process for recycling polystyrene in a dissolving and precipitating procedure. However, use of rapeseed methyl ester may introduce limitations in formulation scope and viscosity range for printing processes.
Thus, there is still a need for varnishes containing polystyrene, where the solvents used to dissolve the polystyrene are more suitable for various printing processes. A need exists to find ways to utilize recycled polystyrene, thereby reducing environmental impact.